An Ethernet PON system connects an OLT (Optical Line Terminal) with a plurality of ONUs (Optical Network Units) by way of optical transmission media, and performs transmission and reception of data by using MAC (Media Access Control) frames to transparently accommodate an Ethernet transfer service therein. The OLT is installed in a communication network carrier station, and each ONU is installed inside or outside of a subscriber's house. Each ONU accommodates one or more terminal devices.
On the other hand, as a method of implementing a multicast transfer via IP communications which have become widely available, there is a method of implementing a multicast transfer by using IGMP (Internet Group Management Protocol) or MLD (Multicast Listener Discovery). IGMP and MLD are protocols used for management of an IP multicast group, and are used by a multicast router that supports a multicast transfer and a terminal device that receives multicast packets. Their operations will be briefly shown below.
The multicast router carries out a multicast transfer of a Query message at regular intervals, and grasps its members by judging whether the multicast router has received a report message from each terminal device. Each terminal device sends a Join message when joining a group, and sends a report message when answering a Query message. Each terminal device notifies a Leave message or a Done message, or stops sending a report message when leaving a group to which the terminal device belongs.
As such a multicast transfer using IGMP or MLD, there are a configuration via a LAN (Local Area Network) including layer 2 switches and a configuration via a PON system.    [Nonpatent reference 1] RFC1112 (Host Extensions for IP Multicasting)    [Nonpatent reference 2] RFC2236 (Internet Group Management Protocol, Version 2)    [Nonpatent reference 3] RFC3376 (Internet Group Management Protocol, Version 3)    [Nonpatent reference 4] RFC2710 (Multicast Listener Discovery (MLD) for IPv6)    [Nonpatent reference 5] RFC3810 (Multicast Listener Discovery Version 2 (MLDv2) for IPv6)
However, in a multicast transfer in a general IP network like a multicast transfer in a configuration via a LAN, as an example of background of the present invention, because transmission of a report message by another terminal device can be detected, only one terminal device is allowed to transmit a report message in response to a Query message.
In contrast, in a configuration via a PON system, as an example of background of the present invention, because a report message transmitted from a terminal device connected to an ONU cannot be detected by any other terminal device connected to another ONU different from the ONU in view of the characteristics of the PON system, in a multicast transfer via the PON system, in response to a Query message report messages are transmitted from all ONUs to which all terminal devices belonging to the corresponding multicast group are connected. As a result, the larger number of ONUs to which all terminal devices belonging to the same multicast group are connected, the larger number of report messages are transmitted to the OLT. Therefore, the possibility that the OLT has failed in getting some report messages because of lack of its processing capability increases.
As a measure of preventing a failure in reception of some report messages from occurring, there is a technique of increasing a robustness variable value to increase the number of times that report messages are retransmitted. However, because this technique further increases the number of report messages which the OLT has to receive, it is adversely effective on a failure in reception of some report messages which results from lack of the processing capability of the OLT.
In order to reduce the possibility that the OLT has failed in getting some report messages because of lack of its processing capability, it is effective to lessen the number of report messages which the OLT has to receive per unit time and it is therefore possible to increase the longest response time included in a Query message. Because the timer included in each terminal device which receives a Query message has an initial value which is a random value equal to or smaller than the longest response time, an increase in the longest response time relatively lengthens the time interval during which report messages are transmitted from terminal devices connected to different ONUs and belonging to the same multicast group, the terminal devices being respectively subordinate to the different ONUs, to the OLT.
A problem is however that because an increase in the longest response time which can be set, but has an arbitrary fixed value causes a delay to occur when a terminal device leaves a multicast group, and this results in a waste of the band due to useless transmission of multicast packets.
It is an object of the present invention to provide a station side device and an optical communication system which prevent a failure in reception of some report messages from occurring resulting from lack of the processing capability thereof, and which reduce the waste of the band due to useless transmission of multicast packets, which is caused by a delay in reporting that a terminal device has left a multicast group, as much as possible.